docs: def prepare_to_render(self, request, contextual_vals, option_vals):
          """
          Before calling ``render_to_template()``, this method is called to give
          the instance opportunity to prepare itself. For example,
          ``AbstractMainMenu`` and ``AbstractFlatMenu`` needs to call
          ``set_max_levels()`` and ``set_use_specific()`` to update the
          ``max_levels`` and ``use_specific`` field values to alternative values
          passed as option values to the calling menu tag.
  
          By default, a reference to the 'contextual_vals' and 'option_vals'
          namedtumples prepared by the class in ``render_from_template()`` are
          set as private attributes on the instance, making those values
          available to other instance methods. ``set_request()`` is also called
          to make the current HttpRequest available as ``self.request``.
          """
          self._contextual_vals = contextual_vals
          self._option_vals = option_vals
          self.set_request(request)